Keeping in mind youll wanna put about 500-800 bucks worth the parts into any bolt action to have a stable reliable platform.
Parts include but are not limited to.
Inner barrel
Hop up
Spring
Spring guide
Sears
Piston

Other stuff to consider
Barrel spacers
Cylender
Cylender head
Scope (a good real steel scope)
"zero" trigger or simaler (this will often come with a piston and you wont have to replace the sears as it will have good (read strong) sears. So instead of 40-60 on sears and 100 on a piston youll be lookin at 250 or so for ghe trigger set and piston)
Sorbo pad